Abstract

A numerical program has been developed to simulate an assembly of inelastic, frictional hard spheres inside a control volume undergoing a steady‐state rapid Couette flow induced by the top and bottom bumpy walls. The bumpy walls are made of hemispheric particles fixed onto flat plates. The flow particles can collide with the wall particles and the exposed flat areas of the walls. The macroscopic flowproperties are found to depend on a number of material and geometric properties of the granules, the bumpy walls, and the control volume. These properties include the overall solids fraction of the system, the height of the shear gap, the wall‐particle concentration, the wall‐particle distribution, the diameter ratio of the wall particle to the flow particle, the coefficients of restitution, the friction coefficients, and the sticking tangential restitution coefficients between the flow particles, the wall particles, and the flat walls. A parametric study is undertaken to examine the effect of some of the interesting factors identified above. A new definition for the slip velocity yields positive values consistently, and it represents a significant improvement over the previous ones. By exposing the flat areas of the bumpy walls for collisions, the transfer of energy and momentum from the driving surfaces to the flow medium can be enhanced. Depending on the wall‐particle distribution, there exist optimal wall‐particle concentrations at which the stresses may be maximized or the slip velocities may be minimized. For hemispheric wall particles arranged in an equilateral triangular lattice, the optimal wall‐particle area fraction for maximizing the stresses is about 0.44 while the one for minimizing the slip velocity is about 0.36. The simulation results also show that there exists for gravity‐free Couette flow of inelastic, frictional spheres a critical solids fraction of about 0.5 beyond which the stresses are found to decrease with increasing solids concentration. In general, there is reasonable agreement between the simulation results for stresses and the experimental measurements.